Since compositions which comprise a reinforcing fiber and a thermoplastic resin (hereinafter optionally referred to “fiber reinforced thermoplastic resin composition”) have lightweight and excellent mechanical properties, they are widely used for sporting goods, aero space uses, and general industrial uses. The reinforcing fibers used for these fiber reinforced thermoplastic resin composition reinforce molded articles according to the uses by various forms. For these reinforcing fibers, metal fibers such as aluminum fiber or stainless steel fiber; organic fibers such as aramid fiber or PBO fiber; inorganic fibers such as silicon carbide fiber or the like; and carbon fibers are used. From the viewpoint of balance between specific strength, specific rigidity and lightweight properties, carbon fibers are suitable, and among them, polyarylonitrile carbon fiber is more suitable.
In order to enhance the mechanical properties of molded articles formed from these fiber reinforced thermoplastic resin compositions, it is necessary that the amount of the reinforcing fibers such as carbon fibers is increased. However, when the amount of the reinforcing fibers is increased, it is often observed that the viscosity of the thermoplastic resin containing the reinforcing fibers is increased and thereby the molding processability are decreased, the appearance of a molded article is deteriorated and moreover, a part not filled with the thermoplastic resin is generated to cause strength deterioration.
As means for improving the mechanical properties without increasing the amount of the reinforcing fiber, there is a method of enhancing the interfacial adhesion between the reinforcing fiber and the thermoplastic resin and thereby improving the mechanical properties of a molded article.
When the reinforcing fiber is carbon fiber, the carbon fiber surface has poor reactivity, so that sufficient interfacial adhesion with the thermoplastic resin cannot be obtained in some cases. For coping with this problem, there is, for example, a method of improving the interfacial adhesion according to chemical bonding or mutual interaction with the thermoplastic resin by subjecting the carbon fiber surface to oxidation treatment, thereby adding a reactive functional group such as carboxyl group, hydroxyl group or carbonyl group.
In usual, after the surface oxidation treatment for the carbon fiber, it is general to apply a sizing agent to a carbon fiber bundle in order to improve the unity of the bundle. As the sizing agent, various compounds have been disclosed. Examples thereof are epoxy resin, urethane resin and polyamide resin.
On the carbon fiber surface on which the surface oxidation has been treated, for example, carboxyl group, hydroxyl group or carbonyl group is generated. However, there is a problem that the interfacial adhesion is insufficient and thereby a molded article having excellent mechanical properties cannot be obtained because the reactivity between these functional groups and the functional group contained in the sizing agent is insufficient.
In recent years, fiber reinforced thermoplastic composite materials have been much noticed and molded articles having excellent mechanical properties have been required with increasing the uses thereof. Furthermore, higher economic efficiency and productivity have been demanded industrially. For example, the lightweight properties and economic efficiency by fiber reinforced composite materials are desired. On this account, lightweight olefin resins, particularly propylene resin has been used for a matrix resin.
However, the propylene resin has poor interfacial adhesion with reinforcing fibers, particularly carbon fiber and thereby it is difficult to prepare molded articles having excellent mechanical properties.
Patent document 1 discloses a sizing agent for inorganic fibers obtainable by using a polypropylene resin modified with an unsaturated dicarboxylic acid or a salt thereof. Patent document 2 discloses a sizing agent containing an acid modified polypropylene resin having an acid value of 23 to 120 mgKOH/g. However, these sizing agents cannot give sufficient interfacial adhesion.
Patent document 3 discloses a carbon fiber on which an ionomer resin is adhered in an amount of 0.1 to 8% by weight. Similarly, patent document 4 discloses a carbon fiber on which two kinds of acid modified polypropylene resins are adhered in an amount of 0.1 to 8% by weight. In any of these documents, the objectives are that the interfacial adhesion between the carbon fiber and the matrix resin is improved by adhering a polymer having affinity with a polyolefin resin to the carbon fiber. However, since a modified olefin component such as the ionomer resin or the self-emulsifying polypropylene resin is adhered mainly, coating layer formation of the resin component is insufficient. Therefore, these documents have insufficient effect in order to actively improve the interfacial adhesion by forming a coating layer on the carbon fiber.
In the prior arts as described above, excellent interfacial adhesion cannot be exhibited sufficiently in molding the polyolefin resin as a matrix. Therefore, the development of a fiber reinforced propylene resin composition having excellent interfacial adhesion and capable of preparing molded articles having excellent mechanical properties has been desired.